The present disclosure relates to optical systems and in particular to an all-reflective focal optical system, such as an all-reflective focal telescope that is derived from the first two mirrors of an afocal three-mirror anastigmat optical system or telescope.
An example of a conventional all-reflective focal three-mirror anastigmat telescope is illustrated by U.S. Pat. No. 4,101,195 to Korsch, entitled “Anastigmatic Three-Mirror Telescope,” the entire contents of which are incorporated herein by reference. This patent discloses a focal three-mirror telescope having an ellipsoidal primary mirror, a hyperbolic secondary mirror, and an ellipsoidal tertiary mirror, with the three mirrors producing an image in a conveniently located finite plane for viewing.
An example of a conventional all-reflective afocal telescope is illustrated by U.S. Pat. No. 3,674,334 to Offner, entitled “Catoptric Anastigmatic Afocal Optical System,” the entire contents of which are incorporated herein by reference. This patent discloses a catoptric anastigmatic afocal optical system comprising two concave paraboloidal reflectors of different powers for the primary and tertiary mirrors and a convex hyperboloidal secondary mirror reflector adapted to receive and recollimate radiant energy.
Focal and afocal telescopes are individually best suited for different imaging tasks. For example, certain imaging tasks such as wide area search functions are preferably done in a vehicle scanning mode using linear focal plane array (FPA) technology. A focal three-mirror anastigmat telescope may be best suited for these tasks. Also, certain other imaging tasks such as point collection for detailed interrogation and characterization are best done behind an afocal optical system, because such tasks use custom instruments with two-dimensional (2D) staring FPA's or internally scanned linear FPAs. An afocal telescope may be best suited for these tasks. Thus, depending on the imaging tasks to be performed, conventionally different telescopes may have to be used.
Table 1 illustrates exemplary data for focal and afocal three-mirror anastigmat (TMA) telescopes each having a 75 cm aperture.
RMSFocalAfocalPeak/ValleyWFE,Cassegrain parameterTMATMAWFE, umumPrimary Mirror Conic (F/1.2)−0.992−1.0023.70.9Primary/Secondary Spacing (in)29.56728.433Secondary Mirror Radius (in)13.77616.082Secondary Mirror Conic−1.715−1.94410.42.1
As will be appreciated, the configuration for the primary and secondary mirrors of a focal and an afocal TMA is quite different in radii, conic departures, and spacings.
An all-reflective optical system constructed to provide both focal and afocal optical functions is desired. U.S. patent application Ser. No. 12/369,976 filed Feb. 12, 2009, incorporated herein by reference in its entirety, discloses an all-reflective afocal telescope that is derived from the first two mirrors of a focal TMA optical system, such as a focal TMA telescope.